Fix https://github.com/python-social-auth/social-core/issues/918

[Fleapse]

Make username always a string to avoid errors in the user pipeline.

Proposed changes

Bugfix #918

Types of changes

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• Release (new release request)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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Other information

I was unable to build the make tests docker image
everything ended in error
failed to solve: process "/bin/sh -c /pyenv.sh" did not complete successfully: exit code: 1

I’m not sure what this is connected with (perhaps due to the blocking of the docker hub in Russia or something else, but because of this I could not run the tests)

upd: dont know how, but after rerun docker compose run social-tests tests works

  py38: OK (47.50=setup[10.21]+cmd[2.90,34.39] seconds)
  py39: OK (44.02=setup[6.38]+cmd[3.03,34.61] seconds)
  py310: OK (42.92=setup[6.09]+cmd[2.95,33.88] seconds)
  py311: OK (44.56=setup[5.40]+cmd[4.35,34.81] seconds)
  py312: OK (48.56=setup[7.95]+cmd[4.62,35.99] seconds)
  congratulations :) (227.62 seconds)

[nijel]

Merged, thanks for your contribution!